Kth - sdr - kit

6/13/2010 KTH - SDR - KIT Page 39
________________________________________________________________________________
2.7 Assembling and testing the VHF-UHF module
The VHF-UHF module consists of a double layer board provided with 2 band filters. One filter with 4
Helicals for the 70 centimeter band (430-440 MHz) and a two coil band filter for the 2 meter band (144-146
MHz) or Weather satellite band (136-138 MHz).
There is also the possibility to skip the filters by connecting jumper J1 and J2 to a 50Ω micro strip line on the
upper side of the board. That gives you the possibility to connect directly the antenna signals via the
SGA4586 to the I/Q demodulator. See figure 2.7-1. This is a valuable option if you will use this receiver as a
test receiver. If there is a possibility of overloading, you can switch on the Attenuator.
Two 50Ω GHz relays RL1 and RL2 are responsible for selecting the VHF or UHF filters, and RL3 connects
the I/Q demodulator to the HF or VHF-UHF filter. Depending of the selected frequency the micro controller
switches via RB3 the +5 Volt supply voltage to the module. This +5 Volt supply voltages also excites RL3
to connect the VHF-UHF module to the I/Q demodulator.
Switching between VHF and UHF is done by the micro controller port RB4.
Mount first the components around the +5 Volt supply voltage. See right below on schematic figure 2.7-1.
VR1 should be mounted isolated on the board.
Testing 5 volt supply voltage.
The supply voltage is now ready for testing. Put the VHF-UHF and also the Synthesizer module (needed for
testing) in the corresponding connector on the motherboard (watch the pin 1 indicator).
Connect your 12 Volt DC power supply and check if the receiver starts-up by looking at the LCD. If
everything looks normal, check if the frequency on the LCD is in the VHF or UHF band. If not, select the
corresponding band by pressing the ‘BAND’ pus button whenever you reached the VHF or UHF band.
The supply voltage is via T3 connected to the board. Check the +5 Volt near R3. To get some room for
measuring, you can remove the HF module.
If the +5 Volt is correct the module can be further assembled with the remaining parts.
Notes:
• To prevent short circuit between the relay stand-off part near the ground pin and the pads on the PCB, it
is recommended to put a piece of cardboard or something similar (0.5 mm) between the relay and the
PCB.
• Diode D2 is drawn reversed on the PCB. In figure 2.7-3 of the manual it is OK.
• R5 and R6 are replaced in the kit by wrong resistors. Use for R5=240 ohm and R6=720 ohm or for
R5=220 ohm and R6=680 ohm.
• The trim condenser C7 makes it possible to adjust the coupling factor of the 2 meter band filter to get a
flat pass band. Because a 3-coil band filter was very difficult to tune, we made the choice to skip L3 and
C8 and use C7 as trim condenser. L3 and C8 are not shown in the schematic but they are on the PCB
drawing. See schematic in figure 2.7-1.
Don’t forget to install the wire bridge below diode D2. See figure 2.7-3
___________________________________________________________________________________________________________
Instruction manual KH-SDR wideband receiver Kit No-1021. All rights reserved KTH-SDR-KIT. Software design by PA0RWE.